Film-like composite structure and producing method thereof
1. Technical Field
The present invention relates to a film-like composite structure having a plurality of minute areas of different Schottky currents or Schottky barrier heights and producing method thereof.
2. Background Art
In a semiconductor device, various kinds of interfaces play a fundamental role for operation of the device. At the interfaces, there are potential variations and occur thermal non-equilibrium states of carriers. Among them, at a interface of a metal and a semiconductor, a potential barrier, namely, so-called Schottky barrier is known to occur. The Schottky barrier has a rectifying action. A rectifying metal-semiconductor junction, so-called Schottky junction has been used as a Schottky barrier diode, a Schottky gate transistor or the like, and constitutes a basis of semiconductor devices.
Although it is very important to control a Schottky barrier formed at a metal-semiconductor interface from design and production of the device points of view, a unified understanding of the formation mechanism of the barriers has not yet been obtained. In addition, upon connecting an electrode to an device, there is a metal-semiconductor interface. Accordingly, a non-rectifying ohmic contact is required to obtain, however, in general, it is extremely difficult to circumvent completely a rectifying barrier. Notwithstanding whether the Schottky barrier is used or the ohmic contact is used, it is essential to control the potential barrier at the metal-semiconductor interface. However, so far, there has been formed an interface level inevitably at the interface, accordingly it has been said that it is very difficult to control the Schottky barrier.
Further, when an interface is being formed, due to lattice defects, existence of impurities, interfacial reactions or the like on a surface of a semiconductor, it is difficult to form a uniform interface, that also makes difficult to control the potential barrier at the metal-semiconductor interface. This is partly due to an insufficient understanding of the formation process of the interface. To solve the aforementioned problem, an atomic level understanding and control of the formation process of the interface are essential.
As described above, so far, as Schottky barrier diodes, Schottky gate transistors or the like, various kinds of electronic devices which employ the Schottky barrier have been used. Those employ only an overall Schottky barrier at the metal-semiconductor interface, accordingly their usage has been restricted to such devices that control a current or an amount of electric charges by the whole interface.
Accordingly, it is demanded to, by controlling a potential barrier at a metal-semiconductor interface, produce an interface of a uniform potential barrier, and in addition to make an interface having areas of nanometer level of different potential barriers exist. If those were materialized, it would be expected that they would enable to produce minute semiconductor devices of nanometer level and develop further new functional devices such as mesoscopic devices or the like. However, until now, such an electronic device has not been found out.
On the other hand, as to the value of the Schottky barrier height itself, even for the same metal-semiconductor junctions, different values are reported depending on the state of the interface or the like. For example, in a NiSi2/Si(111) junction system, between the case where (111) face of Si and (111) face of NiSi2 satisfy a complete epitaxial relation, and the case where they are in a twin relation, different values are reported for the value of the Schottky barrier height. In specific, whereas the former one is 0.65 eV, the latter one is 0.79 eV (R. T. Tung, Phys. Rev. Lett. 52, 461 (1984)). This is considered that there is difference between arrangements of Ni atoms and Si atoms at a interface and, this difference leads to the difference of the Schottky barrier height.
Although the aforementioned report shows that the Schottky barrier height differs depending on the difference of the interface structure, it postulates that the Schottky barrier heights in the measured junction area of the metal-semiconductor are the same. This only shows the difference of the Schottky barrier heights at the different metal-semiconductor junctions. Therefore, this does not show that, in a minute junction area of the metal-semiconductor, a plurality of areas of different Schottky barrier height exist. However, such a structure does not exceed the range of electronic devices using the conventional Schottky barrier.
As described above, the electronic devices of the conventional Schottky barrier make use of only the overall Schottky barrier height at the metal-semiconductor interface and a film structure where interfaces of different potential barrier exist with the area of nanometer level has not yet been obtained. From such a background, upon making semiconductor devices minute at the level of nanometer or realizing a new functional device, it is strongly demanded to control the potential barrier at the metal-semiconductor interface. In addition to preparation of the interfaces of a uniform potential barrier, a film-like composite structure where interfaces of different Schottky current and Schottky barrier height coexist at the area of nanometer level is demanded.
An object of the present invention is to provide, through realization of the control of the potential barrier of the metal-semiconductor interface, a film-like composite structure where a plurality of nanometer level areas of different Schottky currents and Schottky barrier heights exist in a minute area, and a producing method of the same.
A film-like composite structure of the present invention comprises a semiconductor layer having a flat portion, a metallic layer of a thickness of 20 nm or less formed on the flat portion of the semiconductor layer, and an intermediate layer of a thickness of 10 nm or less which is partly interposed between the semiconductor layer and the metallic layer and consisting of an insulator, a metal different from the metallic layer or a semiconductor different from the semiconductor layer. Here, the metallic layer has a first area contacting directly with the semiconductor layer, and a second area where the intermediate layer is interposed between the semiconductor layer and the metallic layer, and the Schottky barrier height is different from that of the first area, the first area and the second area each having an essentially uniform Schottky barrier height in each area, respectively.
In the film-like composite structure of the present invention, the first area and the second area are characterized in that the respective interfaces in the each area have an essentially uniform potential barrier, respectively.
In the film-like composite structure of the present invention, the second area can be disposed partly to the whole area of the metallic layer, for example, depending on the desired device pattern. The second area is formed according to the shape of the intermediate layer. For the specific shape of the intermediate layer, an island-like body of the maximum diameter of 100 nm or less, or a belt-like body of a width of 100 nm or less.
A producing method of a film-like composite structure of the present invention comprises a step of forming, on a semiconductor layer with a flat portion, an intermediate layer of a thickness of 10 nm or less consisting of an insulator, a first metal or a semiconductor different from the semiconductor layer, in an island-like or belt-like shape, and a step of forming, on the semiconductor layer having the intermediate layer, a metallic layer of a thickness of 20 nm or less consisting of a second metal different from the first metal, wherein on the semiconductor layer, a first area where the metallic layer and the semiconductor layer contact directly, and a second area where the intermediate layer is interposed between the metallic layer and the semiconductor layer, and the Schottky barrier height is different from that of the first area, are formed.
Upon forming a metal-semiconductor interface, in addition to the control of, for example, an initial surface of a substrate of a semiconductor single crystal, by controlling the formation of a metallic thin film, an insulator thin film or a semiconductor thin film at an atomic level, a potential barrier at an interface was found to be controllable. The present invention was accomplished based on such a finding.
That is, the density of surface state of a surface of a substrate of a semiconductor single crystal is reduced. Further, the surface of the semiconductor single crystal substrate is flattened at an atomic level, and, for the formation of an intermediate layer and a metallic layer, a molecular beam epitaxy (MBE: Molecular Beam Epitaxy) method, for example, is employed. The intermediate layer and metallic layer, after the initial surface being controlled in an ultra-high vacuum, are formed while controlling a substrate temperature or a vaporization speed. With these, the potential barrier of each interface can be controlled.
Based on the control of the potential barrier of the interface, in each of the first area where a metallic layer and a semiconductor layer contact directly, and the second area where an intermediate layer of a thickness of 10 nm or less is interposed between a metallic layer and a semiconductor layer, the interfaces of the respective areas have a uniform potential barrier, respectively. Schottky barrier height in each area mentioned above can be controlled independently. Then, the first area and the second area, there being a difference of an interface structure whether or not the intermediate layer exists therebetween, can be rendered areas of different Schottky barrier height.
On the other hand, an intermediate layer of a thickness of 10 nm or less consisting of a metal, and an insulator or a semiconductor, by making use of a terrace, for example, on a surface of a semiconductor layer, can be partly formed in a desired pattern such as an island or a stripe. Therefore, including on an intermediate layer like this, by forming a metallic layer to cover the surface of the semiconductor layer, a film-like composite structure having a plurality of areas of which the Schottky barrier height is different at a nanometer level can be obtained.
In such a film-like composite structure, for example, by the following way, the composite state of the minute areas of nanometer level can be confirmed. That is, after a film-like composite structure is produced in an ultra-high vacuum, without breaking the vacuum in the ultra-high vacuum, a ballistic electron emission microscopy (Ballistic Electron Emission Microscopy: BEEM) observation is carried out. Through the BEEM observation, the Schottky current and the Schottky barrier height are measured for each area. By thus practicing, the composite state of the minute areas of nano-level can be confirmed.
The film-like composite structure of the present invention makes use of the different Schottky currents and Schottky barrier heights in the aforementioned the first area and the second area. Thereby, the composite structure can be applied to various kinds of highly integrated functional devices including Schottky diodes or Schottky gate transistors finely fabricated on nanometer level.
For example, a tunnel current is injected from a probe to a metallic layer. Among electrons which reached a surface of the metallic layer, a part of the electrons (ballistic electrons) which reached, without scattering in the metallic layer, an interface between the metallic layer and a semiconductor layer goes over a potential barrier, so-called Schottky barrier, of the metal-semiconductor interface and flows to the semiconductor layer as a Schottky current (also called as a collector current or a BEEM current).
In particular, under a condition of the ballistic electrons sufficiently reaching the interface, with respect to the voltage input to the probe, from approximately the Schottky barrier height, the Schottky current increases drastically. Therefore, when the heights of the Schottky barriers of the first area and the second area are different, depending on the voltage (probe voltage) of the tunnel current flowing to the metallic layer, the current that flows to the semiconductor layer between the respective areas, that is, the Schottky current can be controlled. Further, even when the intermediate layer scatters the ballistic electrons strongly, similarly between the first area and the second area, the Schottky current can be controlled.
The film-like composite structure of the present invention can be applied even to a high-density memory device or the like which carries out reading by use of the difference of the BEEM current values of the nanometer areas. As described above, the film-like composite structure of the present invention has a possibility of developing and applying to various kinds of highly integrated functional devices.